six are required for the violet-red series, while four suffice for the blue-violet hues. There is no known means by which we can measure the proportion of two or more _pigments_ in any given mixture, "because color-effect cannot be measured by the pint of mixed paint or the ounce of dry pigment;"[6] but, fortunately, we have a very exact method, in the color-wheel and Maxwell disks, by which the relative proportions of two or more _colors_ in any mixture may be precisely measured. This method has been used in the painting of every one of the 1115 colors of the present work, by means of one disk to represent each one of the thirty-six colors (both pure and "broken"), together with a black, a white, and a neutral gray disk, the last being a match in color to the gray resulting from the mixture of red, green and violet on the color-wheel;[7] the neutral gray disk, however, being used only for the making of disks for the broken series of colors (′, ′′, ′′′, ′′′′, and ′′′′′) and for the scale of neutral grays (Plate LIII.) These colored disks are slit on one side from center to circumference, and therefore by interlocking two or more they may be adjusted so that either occupies any desired percentage of the whole area, which may be very precisely determined by a scale of 100 segments shown on the outer edge of a larger disk on which the colored disks are superimposed. When connected with the color-wheel and adjusted as may be desired, and then rapidly revolved, the two or more distinct colors resolve themselves into a single uniform composite color, whose elements are shown, in their relative proportion, by the scale surrounding the disks.[8]